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Abstract

New Zealand's South Island is actively deforming as a result of oblique continental
collision between the Pacific and Australian Plates. Within the central portion of the
island, this results in reverse faulting, including earthquake rupture on the previously
unreseached Lake Heron Fault. Because few of these faults have been studied, there is
a major gap in knowledge on the paleoseismicity of the region.
In this thesis, I use structural mapping, deformation analysis, geophysics, topo-
graphic contouring, fault measurements, Monte Carlo simulations, and paleoseismic
trenching, in conjuction with geochronologic approaches including Schmidt hammer
exposure age dating, and radiocarbon dating to characterize the earthquake history
and behavior of the Lake Heron Fault.
Measurements of Pleistocene-Holocene displacement of discrete and distributed de-
formation indicate that along the Lake Heron Fault, total vertical deformation is approx-
imately 20 m since the Last Glacial Maximum. This deformation can be constrained,
using Schmidt hammer exposure age dating, to the last 10:15±2:95 ka, and is consistent
across surfaces which could differ in age by up to 15 ka. This suggests that a period
of quiescence was followed by increased activity, and using Monte Carlo simulations, a
vertical slip rate of 2:25 ± 1:05 mm/yr was calculated. This is more than double pub-
lished vertical uplift rates, though when the fault dip within bedrock (60°) is integrated,
net slip falls within published geodetic slip rates. While the fault was seen to dip 60°
in bedrock, near surface dip measurements indicate a de
ection and shallowing of fault
dip by up to 80% in some locations in Late Quaternary sediments.
Further structural calculations indicate that while the Lake Heron Fault has a strike
(200°), preferential to pure dip-slip motion, which is seen on discrete fault scarps, mi-
crotectonic measurements from crestal grabens show that along the fault, near-surface
stress and localized surface rupture is not representative of the regional stress or the
structure at depth.
From paleoseismic trenching, evidence of 2-3 earthquakes was discovered, and using
radiocarbon dating, the timing of the last events is constrained to the last 2.6 ka,
with the most recent event occurring approximately 0:6 ± 0:2 ka. Large single event
displacements (2:75 ± 0:25 m), earthquake magnitudes (Mw 7:4 ± 0:3), and a short
recurrence interval (1:45±0:42 ka) indicate that the Lake Heron Fault has been highly
active in the Holocene.
Results of this study in conjunction with others also show there is a possibility
the Lake Heron and Forest Creek fault represent an 80 km long, segmented reverse
fault, capable of generating Mw 7.7 earthquakes. However, because of variability in the
timing of paleoearthquakes, the structures more frequently ruptures independently in
magnitude 7.0+ earthquakes. Lastly, I propose that changing crustal stress provides a
possible mechanism for apparent temporal variability in the earthquake recurrence on
the Lake Heron Fault.